August 22, 2012

Most Vulnerable Cities Revealed With Flood Risk Index

A new study, published in the latest online edition of Natural Hazards, suggests that Shanghai is the most vulnerable city to serious flooding.

A team of researchers from the University of Leeds and the Netherlands base their findings on a new method they developed to calculate the flood vulnerability of cities.

The index looks at the likelihood of a city's exposure to a major 'once in a hundred years' flood, along with social and economic factors. It incorporates 19 components, including measures of the level of economic activity in a city, its speed of recovery, and social issues such as the number of flood shelters, the awareness of people about flood risks, and the number of disabled people in the population. Several of the index components also look at the level of administrative involvement in flood management.

The index analyzes the vulnerability to coastal flooding of nine cities built on river deltas: Casablanca, Calcutta, Dhaka, Buenos Aires, Osaka, Shanghai, Marseille, and Rotterdam. The results reveal that the highly prosperous megalopolis of Shanghai, in China, is more vulnerable than much poorer cities such as Dhaka in Bangladesh.

“Vulnerability is a complex issue,” explains Professor Nigel Wright, who led the team from the University of Leeds. “It is not just about your exposure to flooding, but the effect it actually has on communities and business and how much a major flood disrupts economic activity. Our index looks at how cities are prepared for the worst — for example, do they have flood defenses, do they have buildings that are easy to clean up and repair after the flood? It is important to know how quickly a city can recover from a major flood.”

Shanghai's vulnerability stems from its exposure to powerful storm surges and the fact that the land is subsiding as sea levels rise. Although a large population lives along the coast in flood-prone areas, much of the city is poorly prepared with insufficient flood shelters for victims.

“A 1-in-100 year flood in Shanghai would lead to widespread damage, with serious consequences for the city, across China and, through wider economic links, for the whole world,” Professor Wright comments.

The index ranks Dhaka, which sits just meters above current sea levels, rather low because although it is regularly hit by tropical cyclones and floods it has few defenses in place and little resilience. Manila and Calcutta are also highly vulnerable because of their large populations and storm exposures.

Marseille and Rotterdam are also exposed to flood risks, with violent storms, high river levels and significant low-lying areas. However, these cities are listed as least vulnerable because of good flood management infrastructure and tight building regulations for flood-prone areas.

“When a big flood hits you will still get flooding,” says Professor Wright, “but these European cities will bounce back quickly.”

The researchers also used their vulnerability index to assess how climate change would affect the vulnerability of these cities in the future. With sea levels predicted to rise over the next 100 years, the study found that Shanghai and Dhaka will remain the most vulnerable cities in 2100, although the vulnerability of all the cities will increase (doubled in the case of Manila).

“Our index provides a flexible tool for cities to explore how they are currently exposed to flooding and how this may change in the future. It will help them to priorities their flood risk and resilience strategies,” says Professor Wright.